Green liquor pretreatment of lignocellulosic material

ABSTRACT

A continuous process for producing a chemical grade pulp, the process comprising the steps of: (a) steaming lignocellulosic material for a first period of time between 1 and 60 minutes; (b) after steaming, impregnating the steamed lignocellulosic material in an impregnation vessel with green liquor for a second period of time up to 5 minutes and at a temperature between 110° C. and 150° C.; and (c) after impregnation, continuously cooking the lignocellulosic material in a digester to produce a chemical grade pulp.

This application claims the benefit of priority to U.S. application Ser. No. 61/171,109, filed on Apr. 21, 2009, the contents of which are incorporated by their entirety.

BACKGROUND OF THE INVENTION

This invention generally relates to impregnating lignocellulosic material and cooking the impregnated lignocellulosic material in a continuous kraft pulping process.

Previous proposals have been made to use black, green, and/or white liquor as an impregnation liquor during the treatment of lignocellulosic material, including, for example, softwood and hardwood chips. See, e.g., U.S. Pat. No. 5,674,359; U.S. Pat. No. 3,520,773; U.S. Pat. No. 1,691,511; EP 0 810 321 B1; Svedman et al., The Use of Green Liquor And Its Derivatives in Improving Kraft Pulping, Tappi Journal Vol. 81, No. 10, pp. 151-158 (October 1998); Ban et al., Low Capital, High Return Modifications to Kraft Pulping Operations, Tappi 2001 Pulping Conference; Ban et al., The Relationship of Pretreatment Pulping Parameters With Respect to Selectivity: Optimization of Green Liquor Pretreatment Conditions for Improved Kraft Pulping, Paperia ja Puu—Paper and Timber, Vol. 86, No. 2, pp. 102-108 (2004); Ban et al., Fundamental Correlations Between Green Liquor (GL) Pretreatment and Pulp Qualities, 2002 Tappi Fall Conference & Trade Fair; Ban et al., Kraft Green Liquor Pretreatment of Softwood Chips. Part II: Chemical Effect of Pulp Carbohydrates, Journal of Pulp and Paper Science, Vol. 29, No. 4, pp. 114-119 (April 2003); Mao et al., Technical Evaluation of a Hardwood Biorefinery Using the “Near-Neutral” Hemicellulose Pre-Extraction Process, 2007 AIChE Annual Meeting; Lucia et al., Green Liquor Pretreatment of Chips Could Boost Kraft Pulping Efficiency, PaperAge, November/December 2002, pp. 24-26.

Black liquor impregnation processes, for example, are described in U.S. Pat. No. 5,192,396 and U.S. Pat. No. 5,346,591. Additionally, the importance of high sulfidity present in the black liquor and its use in impregnation is described in U.S. Pat. No. 5,660,686.

While black liquor is known to be useful, its use may involve some complications. One such complication may be the change in pH that occurs resulting in the undesirable precipitation of lignin. Another is the mere presence of material in the black liquor that can be detrimental to pulp quality and chemical consumption. It is known that the ratio of sodium sulfide to sodium hydroxide can be important to achieve a high sulfidity liquor.

Previous proposals have also included the use of green liquor as an impregnation medium. But these proposals involve batch processes or require the use of long impregnation times (e.g., much longer than 30 minutes), which requires large and/or multiple vessels to maintain a constant throughput in a commercial continuous process for making chemical grade pulp. These prior approaches have drawbacks, e.g., relating to increased capital costs and maintenance costs.

It has been found that the use of green liquor for a short period of time as an impregnation liquor is advantageous. Green liquor has the advantages of containing the beneficial sulfur compounds present in the black liquor without the presence of other, not desirable solid compounds. Furthermore, the ratio of sodium sulfide to sodium hydroxide may be high.

SUMMARY OF THE INVENTION

In an aspect, the invention generally relates to a continuous process for producing a chemical grade pulp, the process comprising the steps of: (a) steaming lignocellulosic material for a first period of time between 1 and 60 minutes; (b) after steaming, impregnating the steamed lignocellulosic material in an impregnation vessel with green liquor for a second period of time up to 5 minutes and at a temperature between 110° C. and 150° C.; and (c) after impregnation, continuously cooking the lignocellulosic material in a digester to produce a chemical grade pulp

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of a two vessel digester system with impregnation.

DETAILED DESCRIPTION OF THE INVENTION

The terms “black liquor,” “white liquor,” and “green liquor” as used in accordance with their commonly understood meanings in the pulping and papermaking art. For example, black liquor may generally refer to the liquor extracted during or the post-cooking, residual liquor; white liquor may generally refer to a mixture of sodium hydroxide and sodium hydrosulfide; and green liquor may generally refer to the dissolved inorganic smelt from the recovery boiler. Green liquor may include, for example, sodium sulfide, sodium carbonate, and sodium hydroxide.

Lignocellulosic material used in this process may generally be wood chips of either softwood or hardwood varieties or mixtures thereof. It is also possible to use other cellulosic materials such as straw, begasse, etc.

FIG. 1 schematically illustrates a continuous process involving a two vessel system with green liquor impregnation in an impregnation vessel and cooking in a digester. As exemplified, the system may include a high pressure feeder 10 or other pressurizing device such as a pump (or hydrostatic pressure) to raise the pressure of the slurry of lignocellulosic material that feeds an impregnation vessel 11, a high pressure inlet line 12 leading to the top 13 of the impregnation vessel 11, and a return line 17 passing from behind screens at the top of the impregnation vessel 11 to the high pressure feeder 10 or other location in the feed system.

Pre-steamed lignocellulosic material (e.g., lignocellulosic material steamed at atmospheric pressure) may be fed to high pressure feeder 10 or other pressurizing device. Alternatively, lignocellulosic material may be steamed at high pressure after transfer from the high pressure feeder 10 or other pressurizing device before entering impregnation vessel 11 (or even in an initial stage of impregnation vessel 11). The steaming may occur at any pressure as well as possibly at super-heated conditions.

Green, white, and/or black liquor may be added to the line 17 to cause impregnation, as well as to various recirculation lines and systems associated with the digester 30, which may be any digester suitable for cooking lignocellulosic material with liquor. The slurry (lignocellulosic material and liquor) is fed via line 29 to the top of the digester 30, and from in back of the screens at the top of the digester 30 liquid is removed in line 31, being heated by heaters 32.

The digester 30 may be any type of conventional or unconventional digester, regardless of whether hydraulic or vapor phase. The impregnation vessel 11, furthermore, may comprise one or more impregnation zones, e.g., two different zones with first a lower temperature zone and then a higher temperature zone, and possibly with a different level of effective alkali in each of the zones. Furthermore, each of the two or more impregnation zones may involve differing concentrations of green liquor, white liquor, and/or black liquor.

In the impregnation process, alkali diffuses into the fiberwall. This opens up the wood structure and perhaps partially dissolves a portion of the wood during pretreatment. This impregnation may last for a short period (e.g., 1 minute) or a long period (e.g., up to several hours). The impregnation temperature is about 110° C.-150° C., more preferably at about 120° C.

In the impregnation vessel 11, the impregnation zone(s) can be co-current and/or countercurrent (in any combination or permutation). Although illustrated as a separate impregnation vessel, understood that under some circumstances impregnation may be on top of the digester 30. Again the digester 30 may be any suitable type used in the pulp and paper industry.

In an embodiment, there is a continuous process using green liquor to produce a pulp. This process may require fewer chemicals compared to extant processes. The process may generally include these steps: (a) steaming lignocellulosic material; (b) after steaming, impregnating the steamed lignocellulosic material with green liquor in an impregnation vessel; and (c) after impregnation, continuously cooking the lignocellulosic material under conventional conditions, possibly with the addition of white liquor, to produce a chemical grade pulp.

In some embodiments, it may be possible to have two or more further impregnation steps involving green liquor, white liquor, and/or black liquor prior to continuously cooking the lignocellulosic material. Preferably, the green liquor is drained and the impregnated lignocellulosic material is sent immediately to the digester for cooking with liquor (e.g., white liquor) without further impregnation step(s).

In step (a), the lignocellulosic material—such as, for example, softwood or hardwood chips—are steamed (e.g., at a temperature of 100° C. at 1 bar) for a period of 1-60 minutes (and preferable about 15 minutes) with water vapor in the absence of any added chemicals (e.g., in the absence of white liquor, black liquor, or green liquor). Other temperatures and pressures (e.g., 6 bar or 10 bar) may be suitable for the steaming step.

In step (b), the impregnation occurs for a period of time up to 5 minutes (e.g., 1 minute, 2 minutes, or 5 minutes) at high pressures (e.g., 6 bar or 10 bar). The impregnation may occur at a temperature of about 110° C.-150° C., more preferably at about 120° C. After the desired contact time, the free green liquor may be removed prior to step (c).

Using the process described above, applicant believes that a good transfer of sulfidity into the lignocellulosic material from the green liquor might be achieved at the very early stage of treatment, and this could manifest itself in great benefits to the overall cooking process. It is known that sulfidity transfer at an early stage of the process contributes to improve properties of the pulp produced compared to sulfidity transfer at later stages of the process. See, e.g., U.S. Pat. No. 5,660,686 to Henricson et al.

From tests using the described process (as well as a comparative example conducted at 90-100° C.) it was found that at an impregnation temperature of about 120° C. and between 2 and 5 minutes of contact time, the kappa number may be positively impacted by 2 Kappa units. It is believed that pressure was not a significant factor, meaning when impregnation with green liquor occurred at the same temperature, but different pressures, there may be no advantage achieved through varying the pressure.

While at the impregnation temperature, the effect of contact time was explored. Green liquor was allowed to contact the lignocellulosic material for three specific periods of time: 1 minute, 2 minutes and 5 minutes. While there was a difference in the Kappa number when contact occurred for 1 minute versus 2 minutes (2 minutes begin being more favorable), there was no significant difference in the Kappa number when contact occurred for 5 minutes versus 2 minutes. These contact time tests may indicate the green liquor rapidly penetrates the wall of the lignocellulosic material to fill the void space in the lignocellulosic material wall.

This short contact time can be significant, because it may allow for a stable pH condition to exist during impregnation. The short green liquor contact time results in a minimum change in the pH of the lignocellulosic material and green liquor mix before the green liquor may be withdrawn and replaced with white liquor. The relatively stable pH environment may result in a decreased possibility of lignin precipitation from acid soluble lignin. Decreased lignin precipitation may cause fewer side reactions, such as condensation of lignin.

The benefit of the short duration green liquor impregnation as described above may also include a reduction (e.g., of up 40%) in the consumption of Effective Alkali (EA) charge to the cooking process. For example, to achieve a Kappa number 16 in the process without green charge may require an 18% (EA). Using the described green liquor impregnation process a Kappa number of 16 may require a 10% EA change. This significant reduction in the quantity of EA can reduce the size of the causticizing plant as well as reduce the size of the lime kiln facilities. Reduction in these facilities (as well as the obvious reduction in impregnation vessel size) may cause a reduction in the overall energy consumed in the mill as well as a reduction in the pollutants such as carbon dioxide.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A continuous process for producing a chemical grade pulp, the process comprising the steps of: (a) steaming lignocellulosic material for a first period of time between 1 and 60 minutes; (b) after steaming, impregnating the steamed lignocellulosic material in an impregnation vessel with green liquor for a second period of time up to 5 minutes and at a temperature between 110° C. and 150° C.; and (c) after impregnation, continuously cooking the lignocellulosic material in a digester to produce a chemical grade pulp.
 2. The continuous process of claim 1, wherein the lignocellulosic material comprises wood chips.
 3. The continuous process of claim 1, wherein step (a) occurs in the presence of water vapor and in the absence of white liquor, green liquor, and black liquor.
 4. The continuous process of claim 1, wherein the first period of time lasts approximately 15 minutes.
 5. The continuous process of claim 1, wherein step (a) occurs at atmospheric pressure.
 6. The continuous process of claim 1, wherein step (a) occurs at a pressure up to 10 bar.
 7. The continuous process of claim 1, wherein step (b) occurs for a period of time of up to 2 minutes.
 8. The continuous process of claim 1, wherein step (b) occurs for a period of time of up to 1 minute.
 9. The continuous process of claim 1, wherein step (b) occurs at a pressure up to 10 bar.
 10. The continuous process of claim 1, wherein step (b) occurs at a pressure up to 6 bar.
 11. The continuous process of claim 1, further comprising the step of removing free green liquor between step (b) and step (c). 